In the past few years, the general public has become increasingly apprehensive of the impact man-made waste has on the environment. Hence there is a growing interest in developing novel biodegradable (and preferably compostable) plastics from renewable resources.
One particularly interesting candidate for this task is poly(hydroxy carboxylic acid), in particular poly(lactic acid) (PLA), now commercially available on a relatively large scale. The lactic acid is obtained from plants such as corn and sugar cane or other sugar-or starch-producing plants. Not only is PLA obtainable from renewable materials, it is also industrially compostable. For these reasons, there is significant interest in using PLA as a substitute in applications, where petroleum-based thermoplastics have conventionally been used.
Unfortunately, PLA used on its own does not have the same advantageous properties as conventional plastics do. In particular PLA has performance problems related to heat resistance, brittleness and limited flexibility, resulting in poor mechanical strength. On the other hand, polyolefins, such as polyethylene, have much better mechanical and rheological properties. It has been attempted to combine these properties by blending PLA with low density polyethylene (LDPE) to obtain a resin that is at least partially made of materials from renewable resources, but still has acceptable mechanical properties. In the past, compatibilising agents were used. However, this requires an additional industrial step, as well as specific conditions during extrusion. Furthermore, the addition of compatibilising agents is expensive and changes the properties of the desired product. Thus both the compatibilising agent and the by-products change the properties of the desired end product, be it a film, fiber or moulded object.
There are various examples in the prior art where attempts have been made to blend certain polyolefins with PLA.
EP 1 777 263 A also teaches mixing polyolefins with PLA by using a compatibiliser, wherein the compatibiliser is a hydrogenated, diene-based polymer containing at least one functional group selected from carboxyl group, acid anhydride group, epoxy group, (meth)acryl group, amino group, alkoxysilyl group, hydroxyl group, isocyanate group and oxazoline group. The polyolefin is a polymer obtained by polymerizing ethylene and/or at least one kind of alpha-olefin using either the high-pressure method or the low-pressure method. Examples of the alpha-olefin includes alpha-olefins of 3 to 12 carbon atoms, such as propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-ethyl-1-pentene, 1-octene, 1-decene, 1-undecene and the like.
US 2005/0192405 A discloses a polymer alloy of PLA and polyolefins. The two components are made miscible by including a polyalkylacrylic ester and/or a polyvinyl ester, as well as a block copolymer of a polyalkylacrylic ester and a polyolefin and/or a block copolymer of a polyvinyl ester and a polyolefin. The polyolefin described is either a polyethylene obtained by a radical polymerisation mechanism or a polyethylene or polypropylene obtained via cationic addition polymerisation mechanisms using Ziegler-Natta catalysts.
Each of these prior arts demonstrates the requirement of compatibilisers to obtain acceptably homogeneous blends of polyolefins with PLA.
JP 2007 063435 A discloses blending PLA with polyethylene comprising vinyl acetate comonomer, also known as EVA. However, the content of vinyl acetate monomer is at least 30 wt %.
U.S. Pat. No. 5,726,220 discloses a blend of PLA and EVA, wherein EVA comprises at least 30 wt % of vinyl acetate comonomer.
The problem with the blends in these pieces of prior art is that the polyethylenes have a high content of vinyl acetate comonomer. The advantageous properties of polyethylene are lost, because as the content of vinyl acetate increases, EVA becomes more and more elastic and less and less crystalline. Transparency increases, but so does its permeability to gas, moisture and oils. On the other hand, if it were possible to achieve homogeneous blends of PLA with an EVA comprising less than 25 wt %, and in particular less than 20 wt % of vinyl acetate comonomer, these would provide much tougher, more resilient resin compositions.
It is thus an object of the invention to develop a resin, which is at least partially obtainable from renewable resources, and has better mechanical properties than hitherto known blends of polyethylene with resins obtainable from renewable resources.
It is additionally an object of the invention to develop a resin, which is at least partially obtainable from renewable resources, but has higher stiffness than pLDPE (e.g. EVA) resin.
It is also an object of the invention to develop a resin, which is at least partially obtainable from renewable resources, and has better mechanical properties than poly(hydroxy carboxylic acid) resins.
Furthermore, it is an object of the invention to blend LDPE with poly(hydroxy carboxylic acid)s, such as PLA, without having to use additional compatibilising agents to obtain homogeneous blends.
It is further an object of the invention to develop a resin that has better gas barrier properties than LDPE.
It is also an object of the invention to develop a resin with better surface tension properties than LDPE.
It is also an object of the invention to find a resin at least partially obtainable from renewable resources that can be used in blown, extrusion, cast and/or coextruded films.
At least one of the above objects is achieved with the implementation of the current invention.